A Pressure Sensitive Adhesive (PSA) is an adhesive whose coating on the surface of paper or film makes it permanently tacky at room temperature and enables the surface to adhere readily on a substrate under light and brief pressure. PSAs are largely used for the manufacturing of labels, in particular paper based labels, which are fixed on articles for information presentation purposes (such as bar code, name, price) and/or for mere ornamental purposes.
PSAs are generally applied by continuous coating processes on a large surface of a support layer, made of a base material (also called facestock) which consists of paper or of a plastic film. The adhesive layer which coats this support layer is protected by a release liner consisting of, for instance, siliconized paper. The resulting laminated system is generally produced so as to be wound up on itself under the form of large rolls of up to 2 m in width and 1 m in diameter, which is an appropriate dimension for storage and transport.
This laminated system may be subsequently converted into commercially useful labels after further processing steps which include printing and cutting. Such labels are generally designated as pressure-sensitive or self-adhesive labels or PSA labels. After removal of the release liner, these labels may be adhered to a substrate, such as an article to be labeled, generally by means of an automated labeling equipment, for instance on the packaging lines of the end-user's industrial site. PSAs, due to their high tack, enable the manufacturing of self-adhesive labels which adhere in a very short time to substrates, which increases manufacturing output.
A Hot Melt Adhesive (HM) is a solventless and waterless adhesive that is solid at room temperature. HMs are applied in the molten state and solidify when cooled down, thereby forming a bond which secures the two substrates to be assembled. Some HMs are formulated in such a way as to give to the corresponding coated substrate a relatively hard and tackless character. Other HMs result in coated substrates with a relatively soft character and a high tack—these HMs are PSAs which are largely used for the manufacturing of PSA labels. Such adhesives are designated under the name of Hot Melt Pressure Sensitive Adhesive (or HMPSA).
Some of these HMPSAs are designed to provide labels (in particular paper based labels) with high adhesive performance, which can be permanently fixed on the articles to be labeled. Once removed from the release liner and adhered to a substrate, these labels tear on any attempt to remove them. Such HMPSAs are also designated as permanent HMPSA, in contrast to removable HMPSAs which are designed to provide labels which once adhered to a substrate must be able to be removed from the substrate after a residence time, and adhered to it again.
HMPSAs generally comprise a thermoplastic polymer (in particular elastomeric block copolymers) in combination with tackifying resins and plasticizers.
When such HMPSAs are coated onto a support layer (or facestock) made of paper in order to manufacture a label, there is the frequent problem that the color of the paper surface of the facestock opposite the coated surface changes over time, especially during storage at temperatures above room temperature. This change of color may take the form of stains or of a loss of whiteness. While it is believed that this discoloration may be due to some migration of low molecular weight compounds present in the HMPSA such as certain tackifiers, it is certainly undesirable from an aesthetic point of view.
Such drawbacks have been avoided by means of a preliminary treatment of the paper layer before coating the HMPSA, thereby creating an intermediate oil barrier. However this treatment complicates the overall manufacturing process of the laminated system.
U.S. Pat. No. 6,214,935 describes HMPSA compositions characterized by less bleed and staining tendencies. These HMPSAs are based on elastomeric components such as styrenic block copolymers and tackifying resins with softening points ranging from about 35 to 60° C.
WO 99/20708 relates to a HMPSA based on a block copolymer having a high level of diblock which exhibits minimal staining of paper. However this HMPSA is a removable grade whose adhesive performance is insufficient for designing a permanent PSA label.